Referring to FIG. 16, a straight continuous gas carburizing furnace 1A of a known type includes a preheating zone 4a and a carburizing diffusion zone 4b, which are collectively referred to as a heating zone 4 in the following description, a cooling zone 5, and a quenching zone 6. The heating zone 4, the cooling zone 5, and the quenching zone 6 are not separated by partition doors but are integrated. The heating zone 4, the cooling zone 5, and the quenching zone 6 have a temperature gradient therebetween.
More specifically, after being heated in the heating zone 4, a work W is cooled down to a quenching temperature in the cooling zone 5, which is continuous with the heating zone 4, and is retained in the quenching zone 6, which is continuous with the cooling zone 5, for a predetermined period of time in accordance with a carburizing cycle. Consequently, variation in temperature distribution in the furnace is large due to the on-off operation of a heater and also the temperature at the front of a tray differs from the temperature at the rear of the tray so that quality control of the work W is difficult.
In FIG. 16, 7 denotes a tray pusher, 20 denotes a quench oil tank, 21 denotes an entrance door, 22 denotes an exit door, and 32 denotes an inlet chamber (these reference numerals also refer to the same components in the following drawings and description).
The applicant proposed the provision of a separate chamber functioning as a cooling and quenching zone in the aforementioned known integrated type furnace. That is, as shown in FIG. 18, a continuous gas carburizing furnace 1B includes a cooling and quenching zone 6 which is disposed at the rear end of the heating zone 4 and is separated from the heating zone 4 by a partition door 2 to serve as a separate chamber (see Japanese Examined Patent Application Publication No. 62-21866).
With the continuous gas carburizing furnace 1B, a tray pusher 7 pushes a work from an inlet chamber 32 into the heating zone 4 and the work undergoes predetermined heating therein. After that, a side pusher 26 pushes the work to open the partition door 2, which is disposed between the heating zone 4 and the cooling and quenching zone 6 separated from the heating zone 4. Then, the work is transferred to the cooling and quenching zone 6, which is separated from the heating zone 4, and is cooled to and held at a quenching temperature therein during one carburizing cycle. After that, the work is moved to the quench oil tank 20 by an extracting pusher 27 and is quenched regardless of the carburizing cycle.
Unlike the straight continuous gas carburizing furnace 1A, the continuous gas carburizing furnace 1B includes the cooling and quenching zone 6, which is separated from the heating zone 4, so that the work is free from the influences of the temperatures of the works situated in front of or behind it, leading to a great improvement in quenching temperature distribution. Furthermore, the concentration of carbon can be separately controlled in the cooling and quenching zone 6, thereby greatly improving control over the concentration in the atmosphere. Hence, the quality of the work is improved.
Furthermore, unlike the straight continuous gas carburizing furnace 1A, according to the continuous gas carburizing furnace 1B, two doors will not open simultaneously so that any change in the pressure in the furnace is small. Furthermore, immediately after the temperature reaches the cooling temperature, the work is quenched (zero quench), whereby distortion of the work is minimized.
Furthermore, unlike the straight continuous gas carburizing furnace 1A, according to the continuous gas carburizing furnace 1B, it is not necessary to hold the work in the cooling zone 5, which is continuous with the heating zone 4, in order that the temperature of the work is decreased to the quenching temperature, thus reducing the time during which the work is retained in the cooling zone 5. Accordingly, the overall heating time is drastically reduced which, in turn, reduces the heating energy and atmospheric gas. As a consequence, cost reduction is accomplished, exhibiting good economic effects.
Unfortunately, according to the continuous gas carburizing furnace 1B, the cooling and quenching zone 6 is disposed at the rear end of the heating zone 4 as a separate chamber. Moreover, the continuous gas carburizing furnace 1B requires the side pusher 26, the extracting pusher 27 and the like besides the tray pusher 7 and thus has a complicated structure, thereby requiring a larger installation space.
Another furnace is proposed (Japanese Examined Patent Application Publication No. 61-16912). In this furnace, a heating chamber, a carburizing chamber, and a cooling and holding chamber are separated by partition doors and a work is conveyed by roller hearths which are separately provided at the respective chamber. However, such a furnace that employs only the roller hearths to convey the work in the furnace is larger than the tray-pusher type furnace. Furthermore, with this type of furnace, loss of heating energy due to heat dissipation is large, resulting in increased cost.
It is an object of the present invention to provide a heat treatment furnace which exhibits not only various advantages achieved by the continuous gas carburizing furnace 1B or the like but also a shorter lead time (the time during which a work is retained in the furnace) than the straight continuous gas carburizing furnace 1A. Furthermore, the heat treatment furnace of the present invention has a simpler structure as compared to the continuous gas carburizing furnace 1B or the like and requires a reduced installation space.